A wireless transmission apparatus includes an amplifying apparatus that amplifies a signal for transmission from an antenna. The performance of the amplifying apparatus is primarily evaluated on the basis of efficiency and linearity. The efficiency and linearity of the amplifying apparatus are determined by the characteristic of a power amplifier that inputs a power-amplified signal to the antenna. A power amplifier that provides higher efficiency consumes less power. A power amplifier that provides higher linearity linearly amplifies input signals of a wider power range, from small power ones to large power ones.
Power amplifiers currently employed in amplifying apparatuses include what is known as a class-A amplifier. The class-A amplifier produces small distortion and provides high linearity. On the other hand, the class-A amplifier constantly consumes power incidental to a DC bias current, and hence the efficiency is low.
One of the methods of improving the efficiency of the power amplifier is adjusting a drain voltage in accordance with the amplitude of the input signal, on the basis of a transistor characteristic that the efficiency becomes high in a saturation region. Accordingly, the amplifying apparatus includes a drain voltage controller that controls the drain voltage in accordance with the input signal amplitude.
Controlling the drain voltage in accordance with the input signal amplitude enables the efficiency of the power amplifier to be improved. However, adjusting the drain voltage in accordance with the input signal amplitude so as to improve the efficiency makes the transistor operate close to the saturation region, which leads to increased non-linearity of a gain of the power amplifier. The increase in non-linearity of the gain creates a distortion in an output waveform, because of fluctuation of the gain depending on the inputted power. Such a distortion originating from the non-linearity of the gain is called a non-linear distortion.
To compensate for the non-linear distortion of the power amplifier, the amplifying apparatus includes a distortion compensator. The distortion compensator detects the non-linear distortion generated in the power amplifier on the basis of a difference between a value of a part of the output signal of the power amplifier fed back to the input side and the input signal. The distortion compensator corrects the power amplitude of the input signal on the basis of the non-linear distortion thus detected so as to compensate for the non-linear distortion of the power amplifier, thereby linearizing the characteristic of the power amplifier.
Techniques related to an amplifying apparatus including a drain voltage controller and/or a distortion compensator can be found, for example, in Japanese Laid-open Patent Publications No. 2008-78702, No. 2003-8360, No. 2003-526980, No. 2009-16999, and No. 2005-244935.
In the power amplifier, a non-linear distortion of an output signal that depends on an input signal of a different timing from the timing of the respective input signals is generated, in addition to the foregoing static non-linear distortion. Such a dynamic non-linear distortion of the amplifier arises from a memory effect of the amplifier. The amplifier is composed of transistors. The dynamic non-linear distortion is generated by a parasitic capacitance component and a parasitic inductance component present in a bias circuit between the transistor and the drain voltage Vds. In a circuit where the drain voltage Vds is fixed, a part that reduces a low-frequency impedance can be mounted in the bias circuit, and hence the dynamic non-linear distortion can be suppressed. In contrast, in the case of adjusting the drain voltage Vds, the part for reducing the low-frequency impedance cannot be provided between the amplifier and the drain voltage controller. Accordingly, the parasitic capacitance component and the parasitic inductance component increase in the bias circuit, resulting in an increase in dynamic non-linear distortion. The parasitic capacitance component and the parasitic inductance component present in the circuits of the drain voltage controller also act to increase the dynamic non-linear distortion. For such reasons, a timing for adjusting the drain voltage with respect to an input signal is shifted when performing the adjustment of the drain voltage. Such a shift of the timing is called the memory effect. The extent of the non-linear distortion due to the memory effect dynamically fluctuates, and therefore the non-linear distortion due to the memory effect remains in the output signal of the power amplifier, although the static non-linear distortion can be compensated for by the foregoing distortion compensator.